At one time, telescope mirror blanks were produced in one piece. A glass having a low coefficient of thermal expansion, e.g., a borosilicate glass, was cast around and over refractory cores in a large mold. Removal of the cores on cooling provided a strong integral blank of less weight than a solid body due to the open network.
With a desire for greater instrumental precision came recognition of the need to lessen the effect of ambient thermal change. This led to blanks composed of either a fused silica or a glass-ceramic material having a near-zero coefficient of thermal expansion, that is, zero plus or minus a ten point range.
This development alleviated the problem of ambient temperature change during instrument operation, but had little effect on other problems. A blank was still extremely heavy and, consequently, difficult to maneuver. Further, a lengthy time was required to cast, or otherwise mold, a glass blank, and then carry out further processing steps, such as glass crystallization. However, the most serious problem arose from the need for relatively high glass melting and crystallizing temperatures which were difficult to maintain and control.
In an effort to alleviate forming problems, it has been proposed to produce the plates and supporting core separately. The parts could then be sealed together either by direct fusion or by an intermediate frit seal. However, this still entailed the use of very high temperatures to melt, crystallize and/or seal near-zero expansion materials. Further, fabricating and assembling the core components were tedious operations, and the structures were prone to distort during the processing.